Variable Frequency Drive (VFD) systems with diode rectifier front ends draw discontinuous current from the power system to which they are connected. This results in current harmonic distortion, which eventually translates into voltage distortion. Typically, the power system is robust and can handle significant amount of current distortion without showing signs of voltage distortion. However, in cases where the majority of the load on a distribution feeder is made up of VFDs with rectifier front ends, the current distortion becomes an important issue. When such non linear load is not distributed equally among the phases of a three-phase AC system, there is imbalance in the harmonic current flowing in each phase. Imbalanced current-flow in the AC system results in imbalanced voltage drop across the various power delivery equipment. The result is an imbalanced voltage across the phases. In a wye connected system with its neutral grounded to the power system ground, this imbalanced voltage causes zero sequence currents to flow into the earth ground. Current flow is always associated with power loss due to resistive component in the neutral connection. Consequently, unbalanced current in the phases of a three-phase AC system creates neutral current, which in turn causes losses in the neutral conductors that often run hot. In addition, the ground current can corrode or hasten corrosion of the large number of underground conduits used in industrial plants to carry many different things important to various industrial processes.
Traditional three-phase power transformers are constructed using strip Si based steel, often called transformer steel. Depending on the application, some are grain oriented Si steel and others are non grain oriented Si steel. In all cases, the basic construction constitutes forming two E shaped thin laminations that are placed on top of each other. The resulting structure yields two windows where three distinct coils capable of carrying current are placed. With this structure, the flux path of the flux produced by the current flowing in the middle coil is different from the flux path of the flux produced by the end coils. This inherent imbalance in the flux paths adds to the already existing voltage imbalance in the AC system due to the reasons discussed earlier. In other words, use of such transformers in the presence of a highly non linear load environment can create more current distortion and lead to more loss in the AC power system. One solution to this problem is to employ three single-phase power transformers. However, the overall cost and the overall size for housing the three single-phase power transformers is always much higher than a traditional single three-phase transformers. Hence, the use of three single-phase AC transformers to achieve a balanced three-phase AC source is rare.
Use of stripped laminations for constructing transformers is also associated with higher power loss since the flux travelling on one lamination needs to jump to the other lamination to continue its closed loop path. Since the flux has to cross a thin air barrier, this results in power loss across this type of distributed and unintentional air gaps found throughout the structure.
Grid-connected transformers run hotter under harmonic loading. Harmonics can have a detrimental effect on emergency generators, telephones and other electrical equipment. When reactive power compensation (in the form of passive power factor improving capacitors) is used with non-linear loads, resonance conditions can occur that may result in even higher levels of harmonic voltage and current distortion thereby causing equipment failure and disruption of power service.
There are many ways of reducing the total current harmonic distortion (THD) caused by VFDs. Multi-pulse techniques are popular because they do not interfere with the existing power system from resonance point of view and they are robust and perform well.
Harmonic distortion concerns are serious when the power ratings of the VFD load increases. Large power VFDs are gaining in popularity due to their low cost and impressive reliability. Use of large power VFDs increases the amplitude of low order harmonics that can significantly impact the power system. In many large power installations, current harmonic distortion levels achievable using twelve-pulse techniques are insufficient to meet the levels recommended in IEEE Standard 519-1992. As a result eighteen-pulse VFD systems are proposed to achieve superior harmonic performance compared to the traditional twelve-pulse systems.
A typical three-phase full bridge rectifier is said to be a six-pulse rectifier because there are six distinct diode pair conduction intervals in one complete electrical cycle. In such a six-pulse rectifier with no DC bus capacitor, the characteristic harmonics are non-triplen odd harmonics (e.g., 5th, 7th, 11th, etc.). In general, the characteristic harmonics generated by a semiconductor rectifier is given by:h=kq±1  (1)
where h is the order of harmonics; k is any integer, and q is the pulse number of the rectifier (6 for a six-pulse rectifier). The per unit value of the characteristic harmonics present in the theoretical current waveform at the input of the semiconductor converter is given by 1/h. In practice, the observed per unit value of the harmonics is much greater than 1/h. From these observations, it is clear that increasing the pulse number from six to either twelve or eighteen will significantly reduce the amplitude of low order harmonics and hence the total current harmonic distortion.
The eighteen-pulse systems have become economically feasible due to the recent advances in autotransformer techniques that help reduce the overall size and cost and achieve low total current harmonic distortion. When employing autotransformers, care should be taken to force the different rectifier units to properly share the current. The eighteen-pulse configuration lends itself better in achieving this goal compared to the twelve-pulse scheme.
For eighteen-pulse operation, there is a need for three sets of three-phase AC supply that are phase shifted with respect to each other by twenty electrical degrees. Typically, this is achieved using a four winding isolation transformer that has one set of primary windings and three sets of secondary windings. One set of secondary winding is in phase with the primary winding, while the other two sets are phase shifted by +20 electrical degrees and −20 electrical degrees, respectively, with the primary. This arrangement yields three phase-shifted supplies that allow eighteen-pulse operation. The use of a DC link choke is optional. The leakage inductance of the transformer may be sufficient to smooth the input current and improve the overall current harmonic distortion levels. One disadvantage of this scheme is that the phase-shifting isolation transformer is bulky and expensive.
Instead of using ±20 degree phase-shifted outputs from an isolation transformer for eighteen-pulse operation, a nine-phase supply can be used, where each phase lags the other by 40 electrical degrees. U.S. Pat. No. 5,455,759 shows a nine-phase AC supply using a wye-fork with a tertiary delta winding to circulate triplen harmonics.
Though the size of the autotransformer is much smaller than an equivalent isolation transformer, most autotransformer schemes require the use of additional series impedance to smooth the input AC current. The rating of the transformer is about 60% of the rating of the load. The rectified output voltage from a balanced nine-phase output is about 1.14 times higher (14% higher) than that obtained from a three-phase six-pulse AC to DC rectifier as noted in U.S. Pat. No. 5,124,904. This requires modifying the basic three-phase to nine-phase converter using more windings and alternate paths to provide the needed step down action necessary to use the resulting topology on an existing three-phase AC to DC rectifier system.
Many topological modifications including the ones in U.S. Pat. Nos. 5,124,904, 5,455,759, 5,619,407, 6,525,951 B1, etc have been employed to overcome the higher rectified voltage issue. However, the extra stub and teaser windings add cost and complexity to the structure.
U.S. Pat. No. 5,124,904 shows a nine-phase AC supply using a delta-fork that does not require any additional delta winding. In this configuration, the average DC output voltage is about 14% higher than that obtained using a standard six-pulse rectifier scheme. This can potentially stress the DC bus capacitors and the IGBTs in the inverter section of a VFD. In order to overcome this, additional teaser windings are used. These windings not only add cost and increase the overall rating of the transformer, but also cause imbalance that results in higher than normal circulating currents in the delta windings, which need to be accommodated. The harmonic performance is good but the overall size is large with rated current flow through the teaser windings.
In order to overcome the 14% higher average DC bus voltage observed in the previous configuration, a modification of the configuration was proposed in the U.S. Pat. No. 5,619,407. The harmonic performance is similar and the average DC bus voltage is equal to that observed in six-pulse rectifiers. Similar to the previous configuration, the stub winding currents are high and the teaser winding needs to carry rated load current making the overall transformer big in size and expensive to wind.
In autotransformer configurations using stub and/or teaser windings, discussed above, the overall size and rating of the autotransformer is higher than the optimal value. Use of stub windings typically results in poor utilization of the core and involves more labor to wind the coils. A polygon type of autotransformer is better than stub type autotransformer from size and core utilization points of view. A polygon type autotransformer is shown in U.S. Pat. No. 4,876,634. This configuration requires the use of inter-phase transformers and input AC inductors to achieve low total current harmonic distortion. The reason is that the outputs are not equally spaced to achieve a nine-phase AC supply as in the previous configurations. The polygon autotransformer provides +/−20° phase shifted outputs to achieve eighteen-pulse operation.
A popular eighteen-pulse autotransformer configuration is shown in U.S. Pat. No. 6,525,951. This configuration is a modified version of the configuration shown in the '759 patent. A delta-connected tertiary winding is included in the wye fork. This construction is called a windmill construction. Initially, the windmill structure was present in each phase and the size of the transformer was still large. The kVA rating was about 60%. By removing the windmill structure from two of the three phases, it was shown that the performance remained equally good. By adopting the modified structure of the '759 patent, the kVA rating of the autotransformer was reduced from 60% to 55%.
In the eighteen-pulse autotransformer systems, the change of current from one conducting diode pair to the other is quite sudden and occurs every forty electrical degrees. The situation is amplified since most autotransformers do not have enough leakage inductance to slow the transition resulting in high di/dt across the rectifier diodes. Though the RMS current rating may not exceed the current rating of the diode, attention should be given to the di/dt of the current through the diodes. It is believed that standard rectifier grade diodes are vulnerable to premature failure.
Some important drawbacks of the topologies discussed in the prior art are that autotransformer based topologies require significant input impedance to smooth the current and reduce the overall input current distortion. Autotransformer techniques utilize complex winding structures, either of the stub-type or the polygon type. These transformers are labor intensive to manufacture and result in poor core utilization. Because of complicated winding structure and the fact that partial turns are not practically feasible to build, the error resulting in rounding off can be significant that influences the final performance. This is one reason why input impedance of significant value is needed to account for such aberrations. Also, the change of current from one conducting diode pair to the other is quite sudden in all autotransformer configurations. This causes higher than normal di/dt stress in rectifier diodes and should be considered while designing systems required to have high reliability. All transformers used are constructed using traditional strip steel based box type construction to house three phase windings. As described earlier, such structures cause more imbalance and require additional inductors to suppress the undesirable effects of such imbalance. The additional passive components occupy space and increase the overall cost of the harmonic mitigation technique.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.